MOLECULES, 



49 





Fi s . 81. 



And if the attraction be least. in the direction of its 

 diagonal planes, the edges will be most easily sepa- 

 rated, as in fig. 81, and a rhombic dodecahedron will 

 be the solid produced by cleavage. 



This supposition of greater or less degree of mole- 

 cular attraction in one direction of the molecule than 

 in another, is consistent with many well known facts 

 in Crystallography. 



- 



The primary form both of corundum, and of car- 

 bonate of lime, is a rhomboid ; and the crystals of 

 these substances may be cleaved parallel to their pri- 

 mary planes, the carbonate of lime cleaving much 

 more readily than the corundum. But the corundum 

 may also be cleaved in a direction a 6, fig. 82, perpen- 

 dicular to its axis, which carbonate of lime cannot be. 



This cleavage would either divide the rhombic mole- 

 cules in half, or,' the cleavage planes would expose the 

 terminal solid angles of the contiguous molecules. 



G 



